The present patent application relates to a method for manufacturing VLSI complementary MOS-field effect transistor circuits (CMOS circuits), wherein p-doped or n-doped tubs are generated in the silicon substrate for the acceptance of the n-channel or p-channel transistors of the circuit. The corresponding dopant atoms are introduced into the tubs by means of multiple ion implantations for the purpose of setting the various transistor threshold voltages. The masking for the individual ion implantations occurs by means of photosensitive resist, silicon oxide, or silicon nitride structures, and the manufacture of the source/drain regions and gate regions as well as the generation of the intermediate and insulating oxide and of the interconnect level is undertaken according to known method steps of MOS technology.
Modern CMOS processes are two-tub processes which are allocated in format either to a p-tub or an n-tub basic design, i.e. the p-tubs or n-tubs are situated in a n-doped or p-doped large-surface silicon substrate as relatively small-surface islands.
Manufacturing the tubs and setting the various transistor threshold voltages (thin oxide and field oxide transistors in n-channel and p-channel regions) occurs by means of multiple, mutually matched ion implantations.
The principal differences and disadvantages of the known CMOS processes lie in the implementation of these process steps. The process sequence of a known n-tub CMOS process as disclosed, for example, in an article by T. Ohzone et al in the U.S. periodical IEEE Trans. Electr. Dev., ED-27, No. 9 (1980), pages 1789 through 1795, incorporated herein by reference, exhibits, for example, the following considerable disadvantages:
1. A separate mask is required for the n-channel field implantation.
2. The polysilicon gate must be shielded from the boron implantation by a photomask.
3. The execution of the two ion implantation steps in 1 and 2 above is only possible upon application of an involved two-stage photolithography process.
Photolithography steps and additional masks, however, considerably reduce the economic feasibility of the process.